1. Field of the Invention
The invention relates to apparatus and methods for removing atherosclerotic plaque from vessels.
2. Description of Related Art
The removal of arterial obturations or occlusions which are due to the formation of atherosclerotic plaque with the aid of ultrasound is one of several, commonly used methods.
In classical endarterectomy, the thrombus located at the occlusion site of the artery, including the inner wall of the artery adhering to the thrombus, the intima and optionally also the media, is removed. This is carried out either as a so-called "open endarterectomy," in which the artery is opened along its length, or as a so-called "semi-open endarterectomy," which starts from two cross-sections in the artery, and removes material by means of so-called ring strippers. Although the open endarterectomy has the advantage that removal of the thrombus can be visually monitored, the sutures of the restored artery are a possible cause of stenoses. In this semi-open endarterectomy, on the other hand, the problem is that--in particular with long obstructions--the mechanical removal by means of a stripper often results in irregular surfaces of the inner walls of the arteries. However, if the media has been only partially removed, or if it was injured, this generally leads to postoperative hyperplasias.
In the case of angioplasty, occluded arteries are expanded with the aid of a balloon-tipped catheter. However, this treatment, which is often performed as a preparatory or additional measure for angioplasty, is to a certain extent always traumatizing. Secondary stenoses due to miointimal hyperplasias are a consequence of this procedure.
Relatively extensive plaques are also removed with the aid of lasers. There is a risk, however, of thermal irritation or injury to the inner walls of the vessel, and perforation of the vessel is also possible.
The use of ultrasound for removing atherosclerotic plaques also has been proposed, for example in U.S. Pat. No. 3,565,062, which describes a hollow, vibrating probe, a catheter coordinated with this probe and provided with slots taking up plaque strips cut from the inner wall of the vessel. U.S. Pat. No. 3,526,219 describes the removal of tissue parts, the ultrasonic energy being used for dividing the tissue into very small parts ("micro-chopping"), which can then be extracted. In both cases--as already stated in U.S. Pat. No. 4,870,953 regarding the method described in U.S. Pat. No. 3,526,219--traumatizing effects on the media may occur on the one hand and, on the other hand, there is uncertainty about the actual size of the removed occlusions.
U.S. Pat. No. 4,962,755 describes a method in which a short, curved ultrasonic probe is introduced into an incision in the artery. Liquid is fed to the vibrating working part of the probe via a small, rigid tube arranged parallel to the probe. Since, on the one hand, the vibrating working part comes to rest between removed plaque or media and adventitia and, on the other hand, on further insertion into the artery, the liquid-providing tube is not pushed into this space, wetting of the vibrating working part takes place only on one side, and wetting is possible only to a limited extent. Thus, after loosening of the plaque, it is necessary to remove the plaque by means of forceps from an incision made in the middle of the occlusion or from two distal or proximal incisions. It is evident that such a method can be successful only in the case of occlusions covering a small area, since otherwise removal of the plaque, which may be only partially detached, by means of forceps can result in the plaque being torn off from the media, or plaque being torn and thus only incompletely removed. Traumatizing of the media cannot be ruled out.
Here too, the problem of satisfactory dissection of the "end-point" is not solved. This problem, which was impossible to solve satisfactorily in the classical endarterectomy, consists in the fact that the transition between the treated inner wall of the artery and that region of the inner wall of the artery in which untreated intima and media remain must be homogeneous and regular, since otherwise this is one of the possible causes of postoperative thromboses.
Austrian Patent No. 340,572 describes ultrasonic probes that are intended for the removal of both small-area and extensive plaques. In the case of the short, bent probe, which is similar to that described in U.S. Pat. No. 4,962,755, the liquid, as one of two alternative possibilities described, is passed via a channel provided in the interior of the probe, through outlet orifices provided at the transition point to the inclined part of the probe, to the vibrating working part of the probe. However, since the working part vibrates transversely, the liquid will not be able to reach the end of the working part but will be sprayed off beforehand. In the case of the long probe too, which is in the form of a ring stripper, the liquid--once again as one of two alternative possibilities--is passed via a channel provided in the probe to the transversely vibrating ring. Apart from the fact that wetting of the ring is once again insufficient, the efficiency of the long probe is increasingly reduced during its insertion into the interior of the artery because the long rod carrying the transversely vibrating ring and having an internal channel moves forward in the narrow intermediate space between adventitia and detached plaque or detached media. The inner wall of the vessel and the detached layers thus exert on the longitudinally vibrating rod a pressure that results in a significant decline in the efficiency of the probe. Finally, since nodes are formed, the transverse vibrations of the ring cease.